A device for loosening sheets of paper lying vertically one on top of another in a stack and that can be used in a combination line (combination machine) is known from DE 1263027. The device comprises an inclined stack support table on which the stack rests. In order to loosen the sheets and perform a pre-separation of the sheets in the stack, the stack support table is caused to vibrate in the offsetting direction and perpendicular to the sheet plane. For this purpose, the stack support table is elastically supported and is indirectly or directly connected to a vibration oscillation generator. The vibration oscillation generator can be adjusted to a suitable oscillation frequency, e.g. 50 Hz. At its output end, the stack support table has a bend in the shape of a sled runner, which is inclined slightly upward. A feed roller and a retarding roller adjoin this runner-like bend, a gap being formed between them, through which single sheets from the paper stack can be guided. Due to the vibrations that are transmitted by the vibration oscillation generator to the stack support table, the uppermost sheets of the paper stack move according to the micro-throw principle in the direction of the gap between the feed roller and the retarding roller. The pulse magnitude and number of vibrations that are transmitted from the vibration oscillation generator to the stack support table can be adjusted such that the sheets of the paper stack are fed in an uninterrupted flow to the gap and can be led through the gap individually. The gap width can be adjusted to the paper thickness. The sled runner-shaped bend at the output end causes a loosening effect when the sheets arrive, wherein the uppermost sheet first reaches the gap in order to be seized by the driven feed roller and pulled through the gap.
A device for removing individual sheets from a stack and for transporting the taken-off sheets is known from DE 1 178 444, comprising a unit for receiving the stack in an upward-tilted position and a conveying device with which the underside of the respective lowest sheet in the stack is seized. To facilitate the takeoff of the lowest sheet, the stack is vibrated by a vibration device that communicates upward and downward vibrations to the stack table. The lowest sheet, which is taken from the stack, is led by the conveying device through a roller.
It has been found in these known devices that the vibration movement that is transmitted by vibrators or vibration generators to the stack support table causes micro jumps of the individual sheets. These micro jumps cause a movement in the direction of the output end of the support table in which a takeoff device, in the form of a roller nip for example, is arranged for taking individual sheets from the stack. The micro jumps can lead to jamming of objects in the roller nip. The vibration movement also leads to a compression of the stack in the lower area. Jams of objects in the roller nip and a compression of the stack, however, frequently lead to malfunctions in removing the sheets, because several sheets are fed simultaneously through the takeoff device and this results in doubled or multiple takeoffs.